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FOCUS

Full-Spectrum Optimized Conversion and Utilization of Sunlight

High utilization of renewable energy is a vital component of our energy portfolio. Solar energy systems can provide secure energy with predictable future costs--largely unaffected by geopolitics and climate--because sunshine is widely available and free. The projects that comprise ARPA-E's FOCUS program, short for "Full-Spectrum Optimized Conversion and Utilization of Sunlight," could pave the way for cost-competitive hybrid solar energy systems that combine the advantages of existing photovoltaic (PV) and concentrated solar power (CSP) technologies.

For a detailed technical overview about this program, please click here.

Arizona State University

ASU is developing a solar cell that can maintain efficient operation at temperatures above 400°C. Like many other electronics, solar panels work best in cooler environments. As the temperature of traditional solar cells increases beyond 100°C, the energy output decreases markedly and components are more prone to failure. ASU's technology adapts semiconducting materials used in today's light-emitting diode (LED) industry to enable efficient, long-term high-temperature operation. These materials could allow the cells to maintain operation at much higher temperatures than today's solar cells, so they can be integrated as the sunlight-absorbing surface of a thermal receiver in the next generation of hybrid solar collectors. The solar cell would provide electricity using a portion of the incoming sunlight, while the receiver collects usable heat at high temperature that can be stored and dispatched to generate electricity as needed.

Arizona State University

ASU is developing a hybrid solar energy system that modifies a CSP trough design, replacing the curved mirror with solar cells that collect both direct and diffuse rays of a portion of sunlight while reflecting the rest of the direct sunlight to a thermal absorber to generate heat. Electricity from the solar cells can be used immediately while the heat can be stored for later use. Today's CSP systems offer low overall efficiency because they collect only direct sunlight, or the light that comes in a straight beam from the sun. ASU's technology could increase the amount of light that can be converted to electricity by collecting diffuse sunlight, or light that has been scattered by the atmosphere, clouds, and off the earth. By integrating curved solar cells into a hybrid trough system, ASU will effectively split the solar spectrum and use each portion of the spectrum in the most efficient way possible. Diffuse and some direct sunlight are converted into electricity in the solar cells, while the unused portion of the direct sunlight is reflected for conversion to heat.

Cogenra Solar, Inc.

Double-Focus Hybrid Solar Energy System with Full Spectrum Utilization

Cogenra Solar is developing a hybrid solar converter with a specialized light-filtering mirror that splits sunlight by wavelength, allowing part of the sunlight spectrum to be converted directly to electricity with photovoltaics (PV), while the rest is captured and stored as heat. By integrating a light-filtering mirror that passes the visible part of the spectrum to a PV cell, the system captures and converts as much as possible of the photons into high-value electricity and concentrates the remaining light onto a thermal fluid, which can be stored and be used as needed. Cogenra's hybrid solar energy system also captures waste heat from the solar cells, providing an additional source of low-temperature heat. This hybrid converter could make more efficient use of the full solar spectrum and can provide inexpensive solar power on demand.

Gas Technology Institute

Hybrid Solar System

GTI is developing a hybrid solar converter that focuses sunlight onto solar cells with a reflective backside mirror. These solar cells convert most visible wavelengths of sunlight to electricity while reflecting the unused wavelengths to heat a stream of flowing particles. The particles are used to store the heat for use immediately or at a later time to drive a turbine and produce electricity. GTI's design integrates the parabolic trough mirrors, commonly used in CSP plants, into a dual-mirror system that captures the full solar spectrum while storing heat to dispatch electricity when the sun does not shine. Current solar cell technologies capture limited portions of the solar spectrum to generate electricity that must be used immediately. By using back-reflecting gallium arsenide (GaAs) cells, this hybrid converter is able to generate both electricity from specific solar wavelengths and capture the unused light as heat in the flowing particles. The particle-based heat storage system is a departure from standard fluid-based heat storage approaches and could enable much more efficient and higher energy density heat storage. GTI's converter could be used to provide solar electricity whether or not the sun is shining.

General Electric

GE is designing and testing components of a turbine system driven by high-temperature, high-pressure carbon dioxide (CO2) to develop a more durable and efficient energy conversion system. Current solar energy system components break down at high temperatures, shortening the system's cycle life. GE's energy storage system stores heat from the sun in molten salt at moderate temperature and uses surplus electricity from the grid to create a phase change heat sink, which helps manage the temperature of the system. Initially, the CO2 remains at a low temperature and low pressure to enable more efficient energy storage. Then, the temperature and pressure of the CO2 is increased and expanded through a turbine to generate dispatchable electricity. The dramatic change in temperature and pressure is enabled by an innovative system design that prevents thermal losses across the turbine and increases its cycle life. This grid-scale energy storage system could be coupled to a hybrid solar converter to deliver solar electricity on demand.

Massachusetts Institute of Technology

MIT is developing a high-efficiency solar cell grown on a low-cost silicon wafer, which incorporates a micro-scale heat management system. The team will employ a novel fabrication process to ensure compatibility between the indium gallium phosphide (InGaP) solar cell and an inexpensive silicon wafer template, which will reduce cell costs. MIT will also develop a color-selective filter, designed to split incoming concentrated sunlight into two components. One component will be sent to the solar cells and immediately converted into electricity and the other will be sent to a thermal receiver to be captured as heat. This will allow the simultaneous availability of electricity and heat. By leveraging the InGaP system, MIT's solar cells will be more tolerant to high temperature operation than today's PV cells and allow recovery of more useful higher temperature waste heat through the micro-scale heat management system. The solar cell and heat recovery system will enable more efficient use of the entire solar spectrum to produce dispatchable renewable electricity.

Massachusetts Institute of Technology

Full-Spectrum Stacked Solar-Thermal and PV Receiver

MIT is developing a hybrid solar converter that integrates a thermal absorber and solar cells into a layered stack, allowing some portions of sunlight to be converted directly to electricity and the rest to be stored as heat for conversion when needed most. MIT's design focuses concentrated sunlight onto metal fins coated with layers that reflect a portion of the sunlight while absorbing the rest. The absorbed light is converted to heat and stored in a thermal fluid for conversion to mechanical energy by a heat engine. The reflected light is directed to solar cells and converted directly into electricity. This way, each portion of the solar spectrum is directed to the conversion system where it can be most effectively used. The sunlight passes through a transparent microporous gel that also insulates each of the components so that the maximum energy can be extracted from both the heat-collecting metal fins and the solar cells. This unique stack design could utilize the full solar spectrum efficiently and enable the dispatch of electricity at any time of the day.

MicroLink Devices

MicroLink is developing a high-efficiency solar cell that can maintain efficient operation at high temperatures and leverage reusable cell templates to reduce overall cell cost. MicroLink's cell will be able to operate at temperatures above 400°C, unlike today's solar cells, which lose efficiency rapidly above 100°C and are likely to fail at high temperatures over time. MicroLink's specialized dual-junction design will allow the cell to extract significantly more energy from the sun at high temperature than today's cells, enabling the next generation of hybrid solar converters to deliver much higher quantities of electricity and useful dispatchable heat. When integrated into hybrid solar converters, heat rejected from the cells at high temperature can be stored and used to generate electricity when the sun is not shining.

Northrop Grumman Aerospace Systems

FSPOT-X: Full Spectrum Power for Optical/Thermal Exergy

Northrop Grumman is developing a dish-shaped sunlight-concentrating hybrid solar converter that integrates high-efficiency solar cells and a thermo-acoustic engine that generates electricity directly from heat. Current solar cells lose significant amounts of energy as heat, because they do not have heat storage capability. By integrating a high-temperature solar cell and thermo-acoustic engine into a single system, thermal energy losses are minimized. The thermo-acoustic unit, which was originally designed for space missions, converts waste heat from the solar cell into sound waves to generate electricity using as few moving parts as possible. The engine and solar cell are connected to a molten salt thermal storage unit to store heat when the sun shines and to release the heat and make electricity when the sun is not shining. Northrop Grumman's system could efficiently generate electricity more cheaply than existing solar power plants and lead to inexpensive, on-demand electricity from solar energy.

Tulane University

Hybrid Solar Converter with Integrated Thermal Storage

Tulane University and its partners are developing a hybrid solar energy system capable of capturing, storing, and dispatching solar energy. The system will collect sunlight using a dual-axis tracker with concentrator dish that focuses sunlight onto a hybrid solar energy receiver. Ultraviolet and visible light is collected in very high efficiency solar cells with approximately half of this part of the spectrum converted to electricity. The infrared part of the spectrum passes through the cells and is captured by a thermal receiver that converts this part of the spectrum into heat with nearly 95% efficiency. The heat is captured by a fluid that is heated to a temperature between 100 - 590°C. This heat energy can be immediately for a variety of commercial and industrial applications that require thermal energy or the heat may be stored in a small-scale thermal energy storage bank that stores energy for conversion to electricity by a heat engine when needed most. Tulane University's system will enable efficient use of the full solar spectrum while storing a large component of sunlight as heat for industrial processes or conversion into electricity at any time of day.

University of Arizona

A CPV/CSP Hybrid Solar Energy Conversion System with Full Use of Solar Spectrum

University of Arizona is developing a hybrid solar converter that splits the light spectrum, sending a band of the solar spectrum to solar cells to generate electricity and the rest to a thermal fluid to be stored as heat. The team's converter builds off the CSP trough concentrator design, integrating a partially transmitting mirror near the focus to reflect visible wavelengths of light onto high-efficiency solar cells while passing ultraviolet and most infrared light to heat a thermal fluid. The visible light is concentrated further before reaching the solar cells to maximize their power output. A thermal management system built into the solar cells allows them to be maintained at an optimal operating temperature and could be used to recover useful waste heat. Hot thermal fluid generated by the converter can be stored and used when needed to drive a turbine to produce electricity. The converter leverages the advantages of both PV and CSP to use each portion of the solar spectrum most effectively. This could enable utilities to provide dispatchable, on-demand, solar electricity at low cost even when the sun does not shine.

University of Tulsa

Double-Focus Hybrid Solar Energy System with Full Spectrum Utilization

The University of Tulsa is developing a hybrid solar converter with a specialized light-filtering mirror that splits sunlight by wavelength, allowing part of the sunlight spectrum to be converted directly to electricity with photovoltaics (PV), while the rest is captured and stored as heat. By integrating a light-filtering mirror that passes the visible part of the spectrum to a PV cell, the system captures and converts as much as possible of the photons into high-value electricity and concentrates the remaining light onto a thermal fluid, which can be stored and be used as needed. University of Tulsa's hybrid solar energy system also captures waste heat from the solar cells, providing an additional source of low-temperature heat. This hybrid converter could make more efficient use of the full solar spectrum and can provide inexpensive solar power on demand.

University of Tulsa

The University of Tulsa is developing a hybrid solar converter that captures ultraviolet and infrared wavelengths of light in a thermal fluid while directing visible wavelengths of light to a photovoltaic (PV) cell to produce electricity. The PV cells can be kept at moderate temperatures while high-quality heat is captured in the thermal fluid for storage and conversion into electricity when needed. The thermal fluid will flow behind the PV cell to capture waste heat and then flow in front of the PV cell, where it heats further and also act as a filter, passing only the portions of sunlight that the PV cell converts most efficiently while absorbing the rest. This light absorption control will be accomplished by including nanoparticles of different materials, shapes, and sizes in the fluid that are tailored to absorb different portions of sunlight. The heat captured in the fluid can be stored to provide dispatchable solar energy during non-daylight hours. Together, the PV cells and thermal energy provide instantaneous as well as storable power for dispatch when most needed.

Yale University

Yale University is developing a dual-junction solar cell that can operate efficiently at temperatures above 400 °C, unlike today's solar cells, which lose efficiency rapidly above 100°C and are likely to fail at high temperatures over time. Yale's specialized dual-junction design will allow the cell to extract significantly more energy from the sun at high temperature than today's cells, enabling the next generation of hybrid solar converters to deliver much higher quantities of electricity and highly useful dispatchable heat. Heat rejected from the cells at high temperature can be stored and used to generate electricity with a heat engine much more effectively than cells producing heat at lower temperatures. Therefore, electricity can be produced at higher overall efficiency for use even when the sun is not shining.

Many of ARPA-E’s technology programs seek to break down silos and build new technological communities around a specific energy challenge. In this video, ARPA-E’s Deputy Director for Technology Eric Rohlfing, discusses how the Full-Spectrum Optimized Conversion and Utilization of Sunlight (FOCUS) program is bringing together the photovoltaic (PV) and concentrated solar power (CSP) communities to develop hybrid solar energy systems. This video features interviews with innovators from the FOCUS project team made up by Arizona State University and the University of Arizona, and showcases how the FOCUS program is combining the best elements of two types of solar to get the most out of the full solar spectrum.

The PVMirror project is a collaborative effort between Arizona State University (ASU) in Tempe and the University of Arizona (UA) in Tucson. Principle Investigator Professor Zachary Holman leads the ASU team in the development of silicon heterojunction solar cells for incorporation into PVMirrors. From his work at EPFL in Switzerland, Zak is an expert in the fabrication, characterization and loss analysis of high-efficiency silicon solar cells.
Professor Mariana Bertoni, also at ASU, contributes her wealth of knowledge in mechanical stress, defect engineering, and module fabrication from her time at MIT and Integrated Photovoltaics to the project; she investigates lamination of solar cells to the curved glass that forms the structural support of a PVMirror.
Professor Roger Angel leads UA’s efforts in designing the optics of PVMirrors, curving glass into parabolic troughs, and measuring PVMirror performance on a sun tracker. Roger is a MacArthur Fellow, the director of UA’s Mirror Lab—which makes the largest optical telescope mirrors in the world—and the founder of concentrating photovoltaic company Rehnu.
Engineer Kate Fisher, techno-economic analyst Rob Stirling, project manager Laurel Passantino, and PhD students Jason Yu, Brian Wheelwright, Rodolfo Peon, and Leon Meng complete the team, and bring previous experience in the photovoltaic, optics, and manufacturing industries.

Dr. Aleksandr Kozlov (PI) is a Senior Research Engineer at the Gas Technology Institute (GTI). Dr. Kozlov has a PhD in Mechanical Engineering from Kazan Aviation Institute, Russia and ScD in Mechanical and Aerospace Engineering from Kazan State Technical University, Russia. He has over 30 years’ experience in heat transfer, hydrodynamics, thermodynamics, combustion, and rocket engines research and teaching, and has been a PI for a number of projects and programs related to various processes in industries such as aerospace, steel, power, automotive, paper, etc. Dr. Kozlov has over 130 publications, including 4 books and has been a recipient of a number of awards and recognition. He is a winner of the 2002 NATO Science Partnership Prize (2002), the National Prize for Outstanding Russian Scientists (2000), the Royal Society (UK’s National Academy of Science) Fellowship (1998), and the National Prize for Outstanding Russian Scientists (1997).
Dr. Roland Winston (Co-PI) is a Distinguished Professor and founding faculty member in the schools of Natural Science and Engineering at University of California at Merced (UC-Merced) and also Director of its Advanced Solar Technologies Institute. Dr. Winston's research and teaching focuses on concentrating solar energy systems and applied nonimaging optics. The concepts developed and the devices invented by Dr. Winston have formed the core of a new technology which carries the promise of making solar energy a truly viable energy source for society. Devices to which Winston's name has become attached include the CPC itself, which is sometimes known as a "Winston solar collector" and "Winston cones", the individual parabolic elements that make up a CPC. Practical applications can be found in photovoltaics, natural lighting of buildings, water heating, space heating and cooling, desalinization, cooking and in the collection of solar UV radiation for the photo-catalytic treatment of contaminated wastewater. Nonimaging optics proved to be an important tool in several other areas including astrophysics, elementary particle physics, infrared physics and vision research. He has had over 200 articles published in scientific journals and over 60 patents.
Dr. Eli Yablonovitch (Co-PI) is a Professor of Electrical Engineering and Computer Sciences at University of California at Berkeley (UC-Berkeley), and Director of the NSF Center for Energy Efficient Electronics Science (E3S), a multi-University Center based at Berkeley. He is a Member of the National Academy of Engineering, the National Academy of Sciences, the American Academy of Arts & Sciences, and is a Foreign Member of the Royal Society of London. In his photovoltaic research, he introduced the 4(n squared) light-trapping factor that is in worldwide use for almost all commercial solar panels. This factor increased the theoretical limits and practical efficiency of solar cells. 4n2 is based on statistical mechanics, and is sometimes called the “Yablonovitch Limit”. His ideas are used in almost all semiconductor lasers concept, including DVD players, red laser pointers, and internet telecommunications. Dr. Yablonovitch is regarded as a Father of the Photonic BandGap concept. He is regarded as a Father of the Photonic BandGap concept.

Jessica Adams, MicroLink Devices - Principal Investigator
Dr. Jessica Adams currently holds the position of Senior Program Manager at MicroLink Devices, Inc in Niles, IL. She received her Ph.D. in Physics from Imperial College London, UK, in 2011. Her research focused on the design and development of high-efficiency nano-structured solar cells for space and concentrator photovoltaic applications, in addition to co-inventing a novel multi-junction solar cell structure lattice-matched to indium phosphide in collaboration with the US Naval Research Laboratory. After graduating, Dr. Adams joined MicroLink Devices as a Senior Research and Development Engineer, where she has managed several programs to develop high-efficiency, ultra-lightweight solar cells and arrays for terrestrial, unmanned aerial vehicle and space applications.
Raymond Hoheisel, US Naval Research Laboratory - Co-Principal Investigator
R. Hoheisel received his Ph.D. degree in physics for work on the characterization and development of III–V multijunction solar cells at the Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany, in 2010. Since 2011, he has been with the George Washington University, Washington, DC, USA as a residing research scientist at the U.S. Naval Research Laboratory, Washington, DC, USA. His current research interests include novel multijunction cell concepts for terrestrial and space applications and related characterization techniques. He has authored and co-authored more than 50 refereed journal publications and conference proceedings in the field of III-V multijunction solar cell degradation, photovoltaic power forecast and multi-junction device characterization techniques.